1. Technical Field
The invention relates generally to molded articles and, more particularly, to a structural member having a strengthened inner rib or reinforced bulwark optionally with an overlay material bonded thereto and a process for producing such a structural member.
2. Prior Art
It is known to provide hollow structural members with a double-walled structure for such applications as load floors in vehicles, shelves, tables, and the like. Conventionally, they should ideally be light in weight, easy to manufacture, inexpensive to produce, and remain relatively rigid in response to the application of reasonable forces.
To impart a decorative effect to a plastic structural member or to improve its feel, an overlay material, e.g., a fabric can be bonded to plastic structural members. This approach can be found in building materials, walls, partitions, doors, and automotive parts, e.g., head rests, arm rests, parcel shelves, and cargo floor lid structural members. For example, Japanese Patent Application Laid-Open No. 6-134842 discloses a hollow plastic article in which reinforcing ribs are provided between walls 80 that defects caused by shrinkage or warping are not evident on the exterior of the walls. In addition, Japanese Patent Publication No. 4-53696 discloses a method for producing a hollow plastic article by blow molding in which an overlay material such as a non-woven fabric is bonded to the exterior face of the article.
Conventional structural members often include stiffening ribs or reinforcing bulwarks that are formed on the inside surface of a bottom wall. The bulwarks have leading faces which are integrally joined to the inside surface of the top wall. See, e.g., U.S. Pat. No. 6,060,144. They are formed by a method in which a hollow plastic molten tube is extruded from the die head of a blow molding machine before being expanded within the mold. This intermediate product is sometimes called a “parison.” The parison and if desired an overlay material are positioned between two halves of a mold. Then the parison is blow molded so that the overlay material is simultaneously bonded to the exterior face of the top wall of the structural member.
One disadvantage associated with this method, however, is that when the leading faces of the reinforcing ribs or bulwarks are pressed against the top wall to form an integral union, portions of the overlay material which are located above the leading faces are pressed hard against the surface of the mold cavity. This action may mar the appearance of the resultant structural member by causing press marks on portions of the overlay material which are subjected to pressure. In addition, if the overlay material is a fabric having outwardly protruding fibers, then the fibers may become matted, which further detracts from the appearance of the resultant structural members. This matting phenomenon is caused by the combination of heat from the parison (which is typically at a temperature of about 200 degrees C.) and the pressure applied against the fabric (typically about 10 atm.). Another disadvantage associated with this method is that the formation of the bulwarks may leave a concave indentation in the bottom wall at the points where the leading faces have been joined to the top wall. As a result, the exterior surface of the bottom wall is not a continuous flat surface.
Among the art located in a search that preceded the filing of this application are the following U.S. references: U.S. Pat. Nos. 5,885,515; 5,194,212; 5,169,470; 5,162,092; 5,144,522; 5,123,688; 5,044,663; 5,040,335; 5,020,687; 4,894,974; 4,857,258; 4,563,374; 4,289,817; and 2,985,968.